In many applications, it is desirable to control the phase of a signal over a wide range of frequencies with nearly linear phase change with input control voltage (or current). It is advantageous to have a constant or nearly constant phase tuning sensitivity over the entire operating frequency range. This is useful when it is necessary to phase modulate signals over a wide range of frequencies with the goal of obtaining a phase modulation independent or relatively independent of frequency for the same input stimulus. In addition it is also desirable to achieve the wideband phase modulation performance with relatively low and flat insertion loss.
Various efforts have been explored for wideband analog phase shifters incorporated with one or more all pass filter (APF). A single all-pass filter by itself would not provide a flat response over a range of frequencies, but by cascading multiple all-pass sections tuned at different center frequencies, the cumulative phase shift response curves results in a composite frequency-phase response that may provide a relatively flat phase over a specified range of frequencies. However, there are drawbacks from cascading multiple all-pass sections centered at different frequencies. Multiple all-pass sections at the same (or similar) center frequency may be required to provide a desired overall phase shift range due to the limited phase shift range for a single all-pass section. This may result in the insertion loss by a factor of two or more.
Accordingly, there is a need for an analog phase shifter for controlling the phase of an RF signal over a wide range of frequencies with nearly linear phase change.